The agency said the previous land speed record by a legged robot was 13.1mph.

Darpa said that the project was part of efforts to develop robots designed to "more effectively assist war fighters across a greater range of missions".

Darpa - which is run by the Pentagon - funded the Massachusetts robotics company Boston Dynamics to build the machine.

"We plan to get off the treadmill and into the field as soon as possible," said the firm's chief robotics scientist, Alfred Rizzi, in a statement.

"We really want to understand what is possible for fast-moving robots."

Animal designs

The robot's movements have been modelled on those of fast-running animals in the wild. The machine is designed to flex and un-flex its back to increase the length of its stride.

The current version is dependent on an off-board hydraulic pump, requiring one of the researchers to hold the tubing out of its way. However, the researchers said a free-running prototype was planned for later this year.

The four-year project, which was commissioned in February 2011, ultimately aims to deliver a robot which can "zigzag to chase and evade", and be able to come to an abrupt halt.

It builds on other models based on animals created by Boston Dynamics including itsBigDog rough-terrain robot, designed to recycle energy from one step to the next, and itslizard-like Rise, which can climb walls, trees and fences by using micro-claws on its six feet and a tail for balance.

Noel Sharkey, professor of artificial intelligence and robotics at the University of Sheffield said the latest achievement was very impressive.

"With faster than human speed, this is a step in the development of a high speed killer that could negotiate a battlefield quickly to hunt and kill," he said.

"The biggest concern about this is that no artificial intelligence system can distinguish between civilians and enemy combatants, and so if this was operating on its own it would fall foul of the laws of war."